Bovine respiratory disease is estimated to result in annual losses to the North American cattle industry in excess of 600 million dollars. Pasteurella haemolytica is recognized as the most important etiological agent in the bovine respiratory disease complex. However, other organisms such as bovine viral diarrhea virus (BVD) and bovine respiratory syncytial virus (BRSV) have been identified as pathogens which may play a significant role in the respiratory disease complex in feedlot cattle. Martin et al. (Can. J. Vet. Res. 53:355 (1989)) reported that seroconversion to P. haemolytica cytotoxin, RSV and BVD were predictive of respiratory disease cases, explaining approximately 69% of all respiratory disease cases in the feedlots.
Vaccines have been generated against many bovine respiratory diseases; however, industry demand to combine several vaccines within a single delivery system has caused increasing concern with the problem of interference and virally induced immunosuppression. The immunosuppressive nature of some antigenic epitopes or antigenic competition may render components of a "protective" vaccine useless or deleterious (Ishikura, H. et al. (1988) Immunol. Rev. 106:93; Mueller, D. H. et al. (1989) Annu. Rev. Immunol. 7:445; Scott, P. et al. (1989) Immunol. Rev. 112:161; Stephens, L. R. (1990) Can. J. Vet. Res. 54:541-544; AralaChaves, M. P. et al. (1988) Am. J. Vet. Res. 49:1955). This immunosuppression is often heterogeneous in outbred species, and is difficult to predict. A method of reducing or eliminating this antigenic interference would be useful in facilitating vaccination through combination vaccines, as well as minimizing morbidity and mortality due to the immunosuppressive effect of a vaccine.